While bottle caps evidencing tampering by means of depending breakaway rings and separable portions are in popular current use, they have many defects and limitations in their manufacture and performance.
Malleable metals such as aluminum have been used to produce breakaway ring caps wherein the lower portion of the skirt contains a peripheral line of weakness having spaced bridges and a ring portion therebelow rolled inwardly onto and under a peripheral locking ring on the bottle neck during capping to engage it in an interference fit. When the container is uncapped, the bridges break, leaving the depending ring attached to the bottle neck. An option to such a cap design is the addition of vertical lines of weakness peripherally spaced, wherein the depending ring ruptures and the ring portions so produced remain affixed to the cap upon its removal from the container. A common defect in the performance of such a cap is that it is entirely dependent on the control maintained in the cap-rolling operation and since this is accomplished on the packaging line it is often poorly controlled with the result that such caps can appear untampered with in cases where they have been. Additional defects and limitations on the use of metal roll-on caps result from sometimes poorly formed and easily stripped threads; high frictional resistance with the bottle neck, especially with those of glass, making both capping and resealing difficult and erratic; frequent loss of seal when the caps are dented, especially noted in pressurized containers; difficulty in uncapping due to poorly defined exterior fluting needed to adequately grip the cap; and their rapidly rising cost. Some of these limitations can be reduced or eliminated by preforming the entire cap prior to capping including a peripheral line of weakness and a ring with an interfering bead therebelow. However, the non-resilient nature of such caps call for special container neck designs and cap skirt designs with their own set of limitations and defects as well as restrictions on the type of container materials which are suitable to coact with them. See, for example, Leftault, U.S. Pat. No. 3,460,703.
As a result of the defects and limitations noted for metal tamper evident caps, a substantial interest has been displayed in the use of plastic caps for such purposes. Plastics caps present prospects for easier uncapping and more reliable resealing, resistance to denting and resultant loss of seal, well defined exterior fluting for good gripping and comparatively lower cost. However, plastic caps introduce a different set of limitations encountered in their manufacture, capping and subsequent performance. Early tamper evident plastic caps possessed depending rings which could be heat-shrunk to engage the bottle neck locking ring. However, this approach imposed on the packager the added bottling line operation and expense of heating and shrinking the ring after capping. In addition, difficulty in precisely controlling the operation also has led to erratic shrinkage with the result that caps appeared tampered with when they had not been.
As a result of objections to the heat-shrunk tamper evident plastic cap, interest has turned to producing the required ring undercut for an interference fit with the neck prior to the capping operation. To date this goal has been achieved only by making undesirable compromises in manufacturing complexity and cost to achieve the undercut as well as by loss of some effectiveness in performing the tamper evidence function as a result of the ring design and the inherent nature and properties of plastic materials. Ring undercuts which are solid, molded-in circumferential beads preclude the use of the more rigid, "non strippable" polymers such as polystyrene and add to manufacturing complexity and cost by requiring molds with complicated part ejection systems which increase mold cost and lower molding productivity. A slitting operation subsequent to molding to provide the line of weakness may also be required. However, its tamper evidence may be marginal because the amount of undercut possible even with such compromises typically is also marginal. That is, the feature may grip the bottle neck sufficiently to perform adequately in normal use but not when confronted by a serious tamperer. The performance of such features is also impaired by the inherent ease of deformation and elongation of the plastic materials used which may allow them to stretch beyond their elastic limit during capping, increasing their diameter and thereby reducing the amount of ring interference with the container neck. Further, such breakaway rings are sometimes engaged with the container neck under a continuing stress which over a period of time produces plastic creep, thereby further reducing the tenacity of the gripping engagement and the integrity of the tamper evidence.
Attempts to avoid the problems associated with producing plastic caps having a solid molded-in undercut on its breakaway ring have led to designs and manufactuing methods which produce a plurality of flaps on the ring interior which may fold out of the way on removal from the mold and which are subsequently folded into an interfering position prior to capping. See, for example, Wilde et al, U.S. Pat. No. 4,497,765, Grussen, U.S. Pat. No. 4,394,918 and Ostrowsky, U.S. Pat. No. 4,470,513. Such approaches have their own manufacturing problems which include complex and costly molds, manufacturing operations subsequent to molding or because of the ready foldability of their interference-producing flaps, their tamper evidence can be readily foiled by a serious tamperer.
Another problem is associated with the fact that otherwise standard bottle neck designs often vary only because their locking ring dimensions vary and prior art plastic caps having preformed breakaway rings are specific to a single bottle neck design. As a result a cap from a given production mold can only be used by packagers employing very much the same bottle neck locking ring design and dimensions. This dictates the use of a larger number of smaller molds resulting in reduced productivity and higher cap manufacturing cost.
Also, it has not generally been feasible to produce metal or plastic caps including depending rings which break into segments and which remain attached to the cap, except by using roll-on metal or heat shrink plastic rings, with the various deficiencies already noted. Plastic caps with molded-in ring undercuts and vertical lines of weakness present problems with premature ring breakage during mold removal or during capping.
Thus, known tamper evident caps with depending breakaway ring features are beset with drawbacks and problems associated with the inherent characteristics of the materials selected; the need to perform difficult-to-control operations on packaging lines; complex and expensive manufacturing tools; low manufacturing productivity and high costs; the fact that the more rigid plastics are restricted in use; and the relative ease with which the feature may be overcome by a serious tamperer and other problems.